Magnetic Resonance Imaging (MRI) has been increasingly used for guiding therapy procedures, due to its distinct advantages when compared to competing imaging modalities. MRI provides excellent soft tissue contrast. Moreover, such procedures are free from ionizing radiations. Not only can the margins of target tumors be defined, it can also characterize respiratory induced organ motion from the same imaging session. Such information can be used for treatment simulation, adaption as well as repeated follow-up exams of the therapy procedure. Furthermore, it can also be helpful for the planning of minimally invasive surgery.
Real-time imaging is the most commonly used MR imaging method for such purpose, due to its broad availability, fast data acquisition and instantaneous image reconstruction on the fly. Typically, real-time images acquired with 2D or 3D acquisition were retrospectively sorted into different respiratory bins to resolve respiratory motion. The major drawbacks of such method, however, are limited spatial resolution (of 2D method) and poor temporal resolution (of 3D method) which hinder the accurate assessment of tumor motion information.
Recently, self-gated 4D (respiratory phase resolved 3D) techniques were developed to address limitations of conventional 4D MRI techniques. Using such self-gated methods, k-space data were continuously acquired under free breathing of patients. Self-gating data were analyzed for deriving respiratory motion information. Such self-gating data could be either periodically acquired using an additional self-gating line, or could be directly extracted from the actual imaging data itself.
FIG. 1 shows an example of 4D MRI sequence using koosh-ball k-space trajectory as an example. Based on motion information extracted from self-gating lines (projections in the superior-inferior direction), k-space data were sorted into different bins with each bin representing a unique respiratory motion status. Direct reconstruction of individual k-space bins results in 4D (respiratory phase resolved 3D) images.
FIG. 2 shows an example of 4D MR images acquired using the above described self-gating method. While soft tissues were clearly depicted, it is also evident that there is a substantial amount of background noise in the image, as well as streak artifact. This becomes increasingly prominent when a larger number of bins was selected in order to fully resolve the respiratory motion, since the number of k-space lines supporting the reconstruction of individual bin scales down with the number of respiratory bins.